Mass algae cultures are generally cultivated in one of two ways. For lesser quantities, such as for use in laboratories, the algae cultures are produced in reactors using fluorescent lamps as a light source in gas sparging for agitation and gas exchange. Commercial quantities of algae cultures are generally produced in outdoor ponds using sunlight and paddle wheels for agitation.
It has been widely recognized in the literature that virtually all plants, including algae, make the most efficient use of light when the light is applied in pulses, commonly referred to as the "flashing-light effect." In outdoor ponds, various methods have been used to enhance natural turbulence in the flow of the medium for the purpose of exposing each algae to a flash of sunlight as it is momentarily near the surface. In laboratory systems using fluorescent lamps, the natural flicker of the lamps and the turbulence produced by the sparging gas also raise photosynthetic efficiency as a result of the flashing light effect.
However, prior studies have shown that the efficiency of light utilization for photosynthesis is much greater with shorter flash durations. In the mid-50s, researchers reported that sunlight flashes of one millisecond, separated by dark periods of 20 milliseconds, produced a growth that was similar to that observed under full continuous sunlight. These results were confirmed by other researchers who gave a broader theoretical interpretation to the results. The researchers report that photosynthesis is 15 times more efficient under ideal flashing light sources than it is under the full sunlight.
Artificially illuminated photosynthesis could certainly be made much more efficient and economically practical with the use of an efficient source of flashing artificial light. Many methods described in the prior art are unfortunately so inefficient that most of the economy is lost in the method of producing the flashes.
One conventional method of achieving a high frequency, short duration flashing light effect involves exposing an algae medium to a continuous light source while agitating the medium so that algae within the medium are effectively exposed to "flashes" of light. However, this level of agitation is prohibitively expensive for large-scale commercial operations.
Alternatively, other flashing light studies have been completed using a continuous light source that is cyclically shuttered to create a flashing effect. However, such mechanical creation of a flashing light is highly inefficient, since the radiant energy expended during the period when the light is shuttered is lost.
Several conventional systems have been developed in an attempt to use fluorescent or other gas discharge lamps for flashing-light sources. One such system, disclosed by U.S. Pat. No. 4,626,065 to Mori, utilizes a continuous light source, such as a fluorescent lamp or sunlight, to create a continuous beam of light that is distributed by a prism to an arrangement of photo radiators immersed in a tank. However, due to losses of radiant energy through the prism and photo radiators, and interruption of light utilization when the beam is switched from one photo radiator to another, the system is not highly efficient.
Other conventional flashing light systems have been developed that utilize gas discharge lamps, including fluorescent lamps, which are supplied with pulses of power from an intermittent power supply. For example, a system disclosed by U.S. Pat. No. 3,876,907 to Widmaver uses a grid control circuit to produce periodic direct current pulses that are supplied to a number of series connected gas discharge lamps. Again, however, this intermittent use of a power supply is not highly efficient.
In my co-pending U.S. patent application Ser. No. 07/855,306, a system including a tank for containing algae in a nutrient medium and a plurality of light source elements that are arranged to illuminate the algae medium is disclosed. The system operates on an AC power source and includes light source elements that are electrically connected to form ".eta." banks of light source elements. Power is supplied to each bank of light sources in a predetermined sequence at regular intervals to substantially evenly supply each bank of light source elements with a series of power pulses while maintaining a substantially continuous load on the power supply. The power pulses are typically half-sinusoids.